Add and update include guards

Include guards in most changed files were missing, I added them keeping
the same style as other SDL files. In some cases I moved the include
guards around to be the first thing the header has to take advantage of
any possible improvements compiler may have for inclusion guards.

src/events/SDL_events_c.h

@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
+#ifndef SDL_events_c_h_
+#define SDL_events_c_h_
+
 #include "../SDL_internal.h"
 
 /* Useful functions and variables from SDL_events.c */
@@ -49,4 +53,6 @@ extern void SDL_QuitQuit(void);
 
 extern void SDL_SendPendingQuit(void);
 
+#endif /* SDL_events_c_h_ */
+
 /* vi: set ts=4 sw=4 expandtab: */

src/events/scancodes_xfree86.h

@@ -18,6 +18,10 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
+
+#ifndef scancodes_xfree86_h_
+#define scancodes_xfree86_h_
+
 #include "../../include/SDL_scancode.h"
 
 /* XFree86 key code to SDL scancode mapping table
@@ -503,4 +507,6 @@ static const SDL_Scancode xvnc_scancode_table[] = {
     /*  80 */   SDL_SCANCODE_F12,
 };
 
+#endif /* scancodes_xfree86_h_ */
+
 /* *INDENT-ON* */